[Feature] Core stochastic process class

[galenseilis]

Have a class implementing general methods for stochastic processes. This may be difficult as some stochastic processes are defined on different principles.

Need to explore design options, but like the random variables it much be possible to sample from them (at the bare minimum).

It would be nice to be able to doing algebra on them. This gets tricky without assumptions about shared sets of parameters values, like whether time points match when adding two time series.